1. Field of the Invention
The present invention is directed to monitoring power of a light emitting device, specifically for a power monitor in the same optical path as the primary beam.
2. Description of Related Art
Light emitting devices such as VCSELs need some form of power control to maintain a constant output. Such control is typically automatically performed by measuring a portion of an output of a light emitting device during operation and using this measurement to control the power supplied to the light emitting device.
Such control typically is achieved when the light emitting device to be controlled is an edge emitting laser because edge emitting lasers output light from two ends thereof. Thus, one output may be used for the desired application, while the other output may be used for the power control.
In contrast, a VCSEL typically only emits light from one surface. Hence, any monitoring of the light must be from the same output as used for the desired application of the VCSEL. VCSELs have lower manufacturing costs and their surface emissions make them easier to integrate with other optical devices than the edge emitting lasers, so the use of VCSELs is very desirable.
Previous attempts to monitor the power of VCSELS typically involve splitting off of a portion of the output beam to use as a monitor beam. Examples of such configurations are disclosed in U.S. Pat. Nos. 5,757,836 and 5,774,486. However, such splitting off obscures part of the beam which may affect the wavefront and imaging, and hence coupling, of the light. Further, if the intensity distribution changes, such as when there is a change in lasing mode, the monitored power may change in a way which does not represent the overall output power of the VCSEL.
Additionally, splitting off of the beam may require the output of the VCSEL to be increased in order to maintain the requisite power level while allowing detection. Previous uses of scattering the beam to create a monitor beam relied on reflection for directing the beam and did not provide an optimal signal to the monitor detector. Further, previous scattering did not insure the entire beam was being monitored.
In the commonly assigned, co-pending related application, U.S. Application Ser. No. 09/386,280, a diffractive was used to effectively monitor an entire beam output by a VCSEL. However, this configuration still requires space for the detector, an additional optical element for the diffraction and alignment of the diffractive element and the detector.
The present invention is therefore directed to power monitoring which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
These and other objects may be realized by monitoring power from a monitor beam which is coextensive with a light beam to be used in an application. Such monitoring may be performed using a photodiode to detect the beam, without requiring a portion of the light beam to take a different path than the remainder of the light beam.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.